Melbourne: 'Weed-like' algae is killing corals in Australia's iconic Great Barrier Reef because of increased carbon dioxide levels in the atmosphere, a new study has warned.

The study conducted by researchers at Griffith University in Australia and colleagues showed that if the world continues with 'business as usual' CO2 emissions important reef building corals will suffer significantly by 2050 and die off by 2100.

Great Barrier Reef. Reuters.

That, researchers said, is because algae will compete for space with corals in the reef, much like a weed and eventually take over. Researchers knew increased CO2 had an effect on seaweed behaviour but have now been able to demonstrate just how this happens. They discovered this is due to an increase in the potency of chemical compounds that poison corals. "This is a major step forward in understanding how seaweeds can harm corals and has important implications for comprehending the consequences of increased carbon dioxide emissions on the health of the Great Barrier Reef," said Associate Professor Guillermo Diaz-Pulido, of Griffith's School of Environment.

"For the algae to grow they need light and CO2, just like any other plant, and because algae in the future would be exposed to much more CO2 in seawater we wanted to know to what extent the CO2 would affect some of the things algae do, the physiology and the interaction with animals," said Diaz-Pulido.

"What we have discovered is that some algae produce more potent chemicals that suppress or kill corals more rapidly. This can occur rapidly, in a matter of only weeks," Professor Mark Hay, from the Georgia Institute of Technology in the US. "If the algae overtake the coral we have a problem which contributes to reef degradation, on top of what we already know with coral bleaching, crown of thorn starfish outbreaks, cyclones or any other disturbance," said Hay.

The research was undertaken at Heron Island, a coral cay on the southern end of the reef using underwater reef experiments and outdoor lab studies. Diaz-Pulido said the study has global impact because one of the seaweeds studied that causes the most damage is a common brown alga species found in reefs worldwide.

"That is a problem because if these algae take advantage of elevated CO2 in seawater that is even more a matter of concern," he said. "The scale of the problem is so big removing a bunch of seaweed from the reef is not going to do much because it just regrows and regenerates, so I think the way to address this really is to reduce the levels of CO2 in the atmosphere," Diaz-Pulido added. The study was published in the journal Scientific Reports.